@tom Thanks for your thoughts Tom, it was new to us as well! We seem to have managed to fix the problem but we're not entirely sure how / why. Our best guess is that was a software glitch with the Microhard radios themselves.
I'm documenting what we found here in case anyone else runs into a similar issue in the future.
We were able to eventually duplicate the same problem on another UAV that had previously been perfectly stable, thus eliminating the VOXL as a potential cause with two separate units showing the issue.
We were testing a variety of settings today (e.g. channel, bandwidth, power of transmitter, which module is master vs slave) just to try and identify if anything made a difference. Eventually after re-setting the settings back to the microhard defaults (which we had done several times already), the problem mysteriously stopped. We've since kept a stable link open on two UAVs for the last ~6 hours without issue, so we suspect that there was some software issue that eventually cleared when we modified the settings. The last change we made was testing which module was master/slave, and after swapping them and then swapping them back the issue went away.
Really puzzling issue, we're just glad it seems to be fixed. As I said, we suspect it was an issue with the radio and not with VOXL. Why it would be perfectly stable with one drone for hours and then have an issue with a different identical drone is beyond me.
You guys do great work, keep it up!